During normal conduction of a human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers causing a depolarization and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (AV) node and a ventricular conduction system causing a depolarization and the resulting ventricular chamber contractions.
Disruption of this natural pacemaker and conduction system as a result of aging or disease can be treated by artificial cardiac pacing. For example, one or more heart chambers may be electrically paced depending on the location and severity of the conduction disorder. Cardiac therapy, such as cardiac resynchronization therapy (CRT), may correct symptoms of electrical dyssynchrony of a patient's heart by providing pacing therapy to one or both ventricles or atria, e.g., by providing pacing to encourage earlier activation of the left or right ventricles. By pacing the ventricles, the ventricles may be controlled such that they contract in synchrony.
Cardiac resynchronization devices operate by either delivering pacing stimulus to both ventricles or to one ventricle in synchrony with atrial activation, namely atrio-ventricular AV synchronous pacing, where ventricular pacing is delivered at a predetermined time interval following each atrial depolarization event at heart rates that are determined to be normal for atrioventricular tracking. The desired result of such pacing stimulus is a more or less simultaneous mechanical contraction and ejection of blood from the ventricles, and in synchrony with atrial contraction, thereby maintaining the AV synchrony and improving heart function. Ideally, each pacing pulse stimulus delivered to a ventricle evokes a response from the ventricle. Delivering electrical stimuli that causes the ventricle to respond is commonly referred to as capturing a ventricle.
Current implantable pacemakers and implantable cardioverter defibrillators (ICDs) are available for delivering electrical stimulation therapies to a patient's heart, such as cardiac resynchronization therapy (CRT). Medical device technology advancement has led toward smaller and smaller implantable devices. Recently, this reduction in size has resulted in the introduction of leadless intracardiac pacemakers that can be implanted directly in a heart chamber. An example of delivering cardiac resynchronization therapy using a subcutaneous device and a leadless pacing device pacing is described, for example, in U.S. Pat. No. 9,789,319, incorporated herein by reference in it's entirety. Timing of the delivery of a pacing therapy is an important consideration for ensuring effective capture during delivery of the pacing therapy by the leadless pacing device, and therefore improvements in effectively sensing and identifying specific desired features of a cardiac signal for timing delivery of a pacing therapy are needed.